EVANSTON, Ill. (CBS) — The deadliest e. coli outbreak in modern history, centered in Germany, has now killed 23 people and sickened more than 2,200 – including four in the United States.

As WBBM Newsradio 780’s Debra Dale reports, a local pathologist says the European outbreak appears to be triggered by a brand new type of e. coli infection.

“We’ve seen this as two separate diseases,” Dr. Tom Thomson, director of the microbiology lab at NorthShore University Health System. “It seems as though the strain that they have in Europe has combined these two mechanisms into one strain.”

LISTEN: Newsradio 780’s Debra Dale reports

Thomson said the source has been elusive.

“These vegetables come from all over the place – literally all over the world,” he said. “Trying to find a specific batch of vegetables that was in a specific store or restaurant where people ate could be difficult.”

Thomson said new cases are still cropping up, but he believes the source will be found.

World Health Organization communicable diseases expert director Dr. Guenael Rodier said the likely culprit could be found in a period of a week, or “we may never know,” CBS News reported.

Officials in Germany first blamed the crisis on Spanish cucumbers, then on German mung bean sprouts, but both conclusions turned out to be incorrect, CBS News reported.

The outbreak has killed 22 people in German, and one in Sweden.